One of the most important tumor suppressor genes, p53, is mutated in more than half of human tumors, p53 functions in controlling the transcription of target genes with profound effects on the cell cycle and apoptosis (a form of cell death). Proteins that interact with and modulate the function of p53 have also proven to be extremely important in tumorigenesis. Our laboratory has been studying the p53-interacting protein, Bbp/53BP2 (recently renamed and referred to here as ASPP2). Under the auspices of the original grant proposal, we have studied the function and regulation of the ASPP2 protein. In published studies, we showed that overexpression of ASPP2 to levels >200 fold above endogenous induce apoptosis whereas lower levels, 5-10 fold above endogenous, sensitize cells to DNA-damage inducing chemotherapy drugs. We also showed that expression of ASPP2 is under complex regulation. Increased levels of ASPP2 sensitized cells to the lethal effects of UV-irradiation whereas decreased levels were protective. Further studies showed that full length ASPP2 preferentially stimulated the transcriptional activation function of p53 on promoters of pro-apoptotic genes. Furthermore, we have found that ASPP2 is degraded in the proteasome under non-stress conditions or cleaved by caspases during apoptosis, generating stable carboxy terminal fragments that have the potential to feedback inhibit the p53-stimulating function of the full length protein. We have also begun to address the in vivo function of ASPP2 through the creation of a knockout mouse model. In this competing continuation, we propose to determine the mechanisms that ASPP2 uses to induce apoptosis in p53 wildtype and deficient cells, the mechanisms that regulate the stability of the ASPP2 protein and the biological relevance of ASPP2 to the stressed cell through fulfilling the following specific aims: AIM 1. Determine the mechanisms of ASPP2 induced cell death AIM 2. Determine the biological function of proteolytic regulation of ASPP2 AIM 3. Define the in vivo biological role of ASPP2 during the cellular stress response.